An electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon influencing charged pigment particles dispersed in a dielectric solvent. An EPD typically comprises a pair of spaced-apart plate-like electrodes. At least one of the electrode plates, typically on the viewing side, is transparent. An electrophoretic dispersion composed of a dielectric solvent with charged pigment particles dispersed therein is enclosed between the two electrode plates.
An electrophoretic dispersion may have one type of charged pigment particles dispersed in a solvent or solvent mixture of a contrasting color. In this case, when a voltage difference is imposed between the two electrode plates, the pigment particles migrate by attraction to the plate of polarity opposite that of the pigment particles. Thus, the color showing at the transparent plate may be either the color of the solvent or the color of the pigment particles. Reversal of plate polarity will cause the particles to migrate back to the opposite plate, thereby reversing the color.
Alternatively, an electrophoretic dispersion may have two types of pigment particles of contrasting colors and carrying opposite charges, and the two types of pigment particles are dispersed in a clear solvent or solvent mixture. In this case, when a voltage difference is imposed between the two electrode plates, the two types of pigment particles would move to the opposite ends (top or bottom) in a display cell. Thus one of the colors of the two types of the pigment particles would be seen at the viewing side of the display cell.
For all types of the electrophoretic displays, the dispersion contained within the individual display cells of the display is undoubtedly one of the most crucial parts of the device. The composition of the dispersion determines, to a large extent, the lifetime, contrast ratio, switching rate and bistability of the device.
In an ideal dispersion, the charged pigment particles in an electrophoretic dispersion must be dispersible and stable in a liquid medium. They also must have proper particle size, charge polarity and density. Therefore, for inorganic pigment particles to meet these requirements, an organic coating over the inorganic pigment particles is often needed. Currently, an organic coating is typically applied by conventional free radical polymerization technique. However, it is difficult to achieve the desired results by such technique. For example, block copolymers, which are useful for surface polarity adjustment and charge generation, can not be made through conventional free radical polymerization. Also, there is limitation on the amount of polymer that can grow on the pigment particle surface through conventional surface free radical polymerization. The amount of polymer that can grow on the pigment particle surface is critical for the pigment particle stability and density match.